200V to 208V Buck-Boost Transformer — Step-up, 3-Phase Delta
See how a 200V to 208V 3-phase delta buck-boost transformer corrects voltage and how to select the proper model.
Choosing a 200V to 208V transformer
200 to 208 volts is the smallest correction in this family and the one that gets argued about, because 200 volts is a legal reading. ANSI C84.1 puts the Range A service minimum for a 208 volt system at 197 volts, so a panel measuring 200 is inside the standard and the utility is under no obligation to change it. A 200V to 208V buck-boost transformer applies roughly a 4 percent boost. Whether that is worth correcting turns on how much tolerance the connected equipment has left, not on whether the supply is compliant. Single-phase and three-phase, 10 through 60 amps.
This pair is chosen by the equipment rather than by the building, so the useful way to sort the applications is by how much input tolerance a load actually has.Plenty of loads absorb 200 volts without help, which is why the correction is often skipped. A general-purpose induction motor nameplated 208-230V is rated against 208, and 200 volts is 3.8 percent low, comfortably inside the plus or minus 10 percent NEMA MG-1 describes.The loads that drive the correction have a narrower window:Packaged equipment offered only in a 208 volt rating, where 208 is the design point rather than the low end of a dual ratingVariable frequency drives, servo amplifiers and soft starters with undervoltage thresholds referenced to a 208 volt inputElevator, hoist and material handling controllers that fault rather than derateLaboratory, imaging and process instruments whose published input window is tighter than plus or minus 10 percentResistive heat, where nothing trips and the only symptom is output arriving about 7 percent short208 volt contactor and relay coils, which sit near dropout and open on a dip the motor would have ridden throughThe supply is one of two situations: a building still distributing a legacy 200 volt nominal system, or a nominally 208 volt service that sits at the bottom of its band all day. Both produce the same reading and the same decision.